The present invention relates to an image carrier cartridge and an image forming apparatus and, more particularly, to an image carrier cartridge and an image forming apparatus of tandem type in which a plurality of image carriers are united into a cartridge which is designed to be detachable from the image forming apparatus, thereby improving the maintainability and allowing the correction of color registration errors.
Generally, a toner image forming means of electrophotographic type comprises a photoreceptor as an image carrier having a photosensitive layer on the outer surface thereof, a charging means for uniformly charging the outer surface of the photoreceptor, an exposure means for selectively exposing the outer surface, uniformly charged by the charging means, to light to form an electrostatic latent image on the outer surface, and a developing means for applying toner as a developer to an electrostatic latent image formed by the exposure means to form a visible image (toner image).
As a tandem-type image forming apparatus for forming a color image, there is an apparatus of a type employing an intermediate transfer belt method in which a plurality of (for example, four) toner image forming means as described above are arranged relative to an intermediate transfer belt (as an example of transfer belts). Toner images formed on the photoreceptor by these unicolor toner image forming means are transferred to the intermediate transfer belt so that the toner images of plural colors (for example, yellow, cyan, magenta, black) are superposed on each other, thereby forming a color image on the intermediate transfer belt.
As a tandem-type image forming apparatus for forming a color image, there is an apparatus of a type employing a paper delivery method in which a recording medium (for example, a paper sheet) is held and carried by a recording medium holding belt (as an example of transfer belts). Toner images formed by a plurality of unicolor toner image forming means are sequentially transferred to the recording medium so that the toner images of plural colors are superposed on each other, thereby forming a color image on the recording medium. In such an image forming apparatus of tandem type, a color registration error, i.e. relative deviation of respective color printed positions, may occur due to a positional error of image carriers or a tolerance of diameter of a photosensitive drum. The color registration error makes the quality of color image poorer. Therefore, means of preventing the color registration error have been taken. An example of such preventing means is disclosed in Japanese Patent Unexamined Publication S63-271275, in which image register marks are formed on a transfer belt by an image forming apparatus and these marks are read out by detecting sensors, thereby correcting a color registration error. That is, the means disclosed in Japanese Patent Unexamined Publication S63-271275 detects a color registration error within the image forming apparatus. Accordingly, even when one or more of color image positions are shifted, the color registration error can be corrected by replacing the corresponding image carrier or the like.
Further, an example of such preventing means is disclosed in Japanese Patent Unexamined Publication H9-304994, in which a plurality of image carriers are integrally supported. In an embodiment thereof, developing means are also supported integrally with the image carriers. Therefore, the positioning accuracy of the image carriers in an apparatus body is improved, whereby the possibility of color registration error is reduced and the maintainability is improved. In addition, there is no possibility of insertion error of image carriers.
The means disclosed in Japanese Patent Unexamined Publication S63-271275 can achieve the correction of color registration error without increasing the precision of parts. However, since the detection sensors for detecting the image register marks are required, there is a problem of increase in cost. Further, there is also a problem that toner is consumed for forming the image register marks and a problem that the image forming operation should be suspended during the positioning operation, thus degrading the operating efficiency of the image forming apparatus. On the other hand, the means disclosed in Japanese Patent Unexamined Publication H9-304994 prevents the color registration error by improving the relative positioning accuracy of the image carriers as described above. However, with the recent increase of resolution and quality of image, required precision of color registration is increased. Therefore, the method of integrating the image carriers as disclosed in Japanese Patent Unexamined Publication H9-304994 is no longer enough to improve the positioning accuracy of the image carriers. While the positioning accuracy of the image carriers may be improved by further increasing the precision of parts, there is a problem that the costs of the parts become extremely high.